1. Field of the Invention
The present invention relates to a thermal printing apparatus having a thermal head which is disposed at right angles to, and is arranged to be movable towards and away from, a medium which is to be printed while it is transported or fed in a predetermined feeding direction. It relates, in particular, to a thermal printing apparatus which is capable of printing on different thicknesses of media to be printed.
2. Description of the Related Art
There has conventionally been known a card printer, as a thermal printing apparatus, which has a thermal head movable towards and away from a medium to be printed while it is transported or fed in a predetermined feeding direction of the medium. In a thermal head of this thermal printing apparatus the heating elements are arranged at right angles to the direction of transporting the medium.
The card printer is a printing device for printing exclusively on a rigid card which is as thick as about 1 mm. On the other hand, the thermal printing apparatus is for printing characters, or the like, on different kinds of media such as plate-like media other than cards, or sheet-like media such as labelling sheets or the like. It is therefore necessary to print on media whose thicknesses vary, for example, from plates of about 3 mm thick to labelling sheets of about 0.1 mm thick.
FIG. 8 shows a printing region of a conventional thermal printing apparatus for printing on media of different thicknesses. A thermal head 50 of this thermal printing apparatus is disposed on a head angle member (or a thermal head holder) 53 at substantially the central portion thereof which is at a distance of 62 mm from a shaft 52 of the head angle member 53. This head angle member 53 is pivotally or swingably supported at one end thereof on the shaft 52. By the rotation of a cam 55 which comes into contact with the opposite end 54 of the head angle member 53 and the urging force of a spring 56, the thermal head 50 is moved in a direction towards, and away from, a medium P to be printed.
However, when the thickness of the medium P changes, the position of the thermal head 50 also changes, and the position of the thermal head 50 varies as shown in FIG. 8. If the thickness of the medium changes by 1 mm with this thermal printing apparatus, the head angle member 53 inclines about 0.9 degree. This inclination of the head angle member 53 leads to a change in the angle of contact of the thermal head with the medium to be printed. As a consequence, this conventional thermal printing apparatus has a disadvantage in that the printing quality becomes poor.